1. Field of Invention
The invention relates to thermographic materials and in particular to positive acting materials. By this is meant that such materials are dark or colored before exposure to heat and turn colorless or at least lighter in color when thermally addressed.
2. Background of the Art
Heat-sensitive imaging sheets have been used for copying, thermal printing, thermal recording, and thermal labeling. Many of these materials involve thermally increasing the reactivity of two or more coponents of a color forming reaction which do not react at normal ambient temperatures. Reactivity is often enhanced by melting of one or both reactants which are physically separated from one another. This separation is usually accomplished either by dispersion in a single coated layer or by being situated in two different coated layers. Several general classes of color forming reactants have been used, of which two common ones are (a) leuco lactone or spiropyran compounds reactable with phenolic compounds (e.g., U.S. Pat. Nos. 3,829,401 and 3,846,153) and (b) heavy metal salts of organic acids reactable with ligands to give colored complexes (e.g., U.S. Pat. Nos. 2,663,654, 3,094,620, 3,293,055 and 3.953,659).
Commercial preference for the transition metal salt class has often resulted from the high stability and near black color of the images produced (U.S. Pat. No. 4,531,141). Of the transition metals used, iron, nickel, and cobalt are common and ferric iron appears to be one of the most preferred (U.S. Pat. No. 2,663,654, 3,953,659, 4,531,141).
No references have been found in the literature to thermographic systems having a positive mode of action as defined above. Such systems require a normally black or heavily colored sheet to be turned substantially colorless by exposure to heat.
Black complexes of iron are numerous but normally do not easily react to a colorless form under heat or other stimulation. In U.S. Pat. No. 4,533,930 organo-thiophosphates, organo-thiophosphinates, and organo-thiophosphonates of ferric iron are disclosed as reacting with chelates to give products of different color, normally darker. In fact many of these thio materials are substantially black themselves.
This present invention shows how such black thio compounds can be used to make positive acting thermographic materials.